Enhanced Thermoelectric Properties of Misfit Bi2Sr2-xCaxCo2Oy: Isovalent Substitutions and Selective Phonon Scattering

Abstract

Layered Bi-misfit cobaltates, such as Bi₂Sr₂Co₂O_y, are the natural superlattice of an electrically insulating rocksalt (RS) type Bi₂Sr₂O₄ layer and electrically conducting CoO₂ layer, stacked along the crystallographic c-axis. RS and CoO₂ layers are related through charge compensation reactions (or charge transfer). Therefore, thermoelectric transport properties are affected when doping or substitution is carried out in the RS layer. In this work, we have shown improved thermoelectric properties of spark plasma sintered Bi₂Sr_2-xCa_xCo₂O_y alloys (x = 0, 0.3 and 0.5). The substitution of Ca atoms affects the thermal properties by introducing point-defect phonon scattering, while the electronic conductivity and thermopower remain unaltered.

Keywords: isovalent substitutions; misfit cobaltates; misfit-layer; thermoelectric properties.